optical parametric oscillators (OPO) are important devices for producing tunable coherent radiation by converting light from a pump source to light at longer wavelengths. The light conversion process can be thought of as the decomposition of a photon into two less energetic photons. In an optical parametric oscillator (OPO), which is a combination of a piece of nonlinear optical material and an optical cavity, a pump beam at frequency .omega..sub.1 produces the gain at the signal and idler frequencies, .omega..sub.2 and .omega..sub.3, where .omega..sub.1 =(.omega..sub.2 +.omega..sub.3). In order to produce optical parametric oscillation, the power of the pump wave has to be above a certain threshold power. Efficient operation of a OPO for frequency conversion may require pump power much higher than the threshold.
During the late sixties and early seventies, a number of laboratories attempted to develop broadly-tunable pulsed OPO's. Their attempts were frustrated by the limitations of materials available then. The available nonlinear crystals at that time include lithium niobate, proustite, and many others. Those nonlinear optical materials have low damage thresholds, and were often destroyed at pump power levels below those needed to efficiently pump the OPO.
In the last few years, there has been a resurgence of interest in optical parametric oscillators resulting from the development of new nonlinear crystals, such as KTP, BBO, and LBO, that combine high damage thresholds and high nonlinear coefficients. Optical parametric oscillators using those new materials are currently being developed in a number of laboratories in the U.S. and abroad. Some degree of success has been achieved in using those materials for the generation of tunable light. For example, Kato and Masutani, in "Widely Tunable 90.degree. Phase Matched KTP Parametric Oscillator," Optical Letters,17, 178 (1992), have reported a pulse-pumped, non-critically phase matched, KTP oscillator that could be tuned over a broad spectral range by varying the wavelength of the pump between 0.7 um and 0.95 um.
Even though the development in material science provided some new nonlinear materials for OPO applications, it would still be desirable to provide a means to use the nonlinear crystals from the 1960's and 1970's in OPO applications without damaging the crystals by the pump source. The pump energy required for OPO's currently available is also somewhat high. As reported by Kato and Masutani, the threshold for their OPO when pumped at 0.865 um with 10 nsec pulse length was approximately 0.7 J/cm2, which corresponded to an incident pump energy of 22 mJ. A pump energy depletion of 30% was observed at an incident pump energy of 70 mJ. Although this kind of pump power can be obtained with a flashlamp-pumped laser as the pump source, it is too high to reach easily with a diode-pumped solid state laser. If pulses shorter than 10 nsec are used to pump the OPO, the threshold will be even higher, and higher pump energy will be required to achieve the same efficiency of conversion.
The threshold pump requirement of an OPO with a pulsed pump is higher than the threshold of the same OPO pumped with a CW source. In the prior art when the set up involves only pumping an OPO to get an output, the gain of an OPO is only present for the duration of the pump pulse. The threshold in such a device is determined partially by intracavity losses and output coupling (which completely determines the CW threshold), the length of the pump pulse, and the time needed for the OPO field to build from noise. If the device is pumped with short pulses, e.g. pulses shorter than 10 nsec, and in many cases even shorter than 1 nsec, the threshold power required by the OPO from a pulses pump source could be several times that for a CW pump. This corresponds to the case where the OPO just starts to operate when the pump pulse is already ending. In order for the OPO to operate efficiently, it is necessary to shorten the build up time further by increasing the pump energy. For these reasons, prior art pulse-pumped OPO's must be pumped several times (between 5 and 10) above the threshold in order to extract energy from the pump pulses.
One method which has been used in the prior art to reduce the threshold of OPO's pumped by a pulsed pump source is "injection seeding." A source of a "seeding wave," which has a wavelength generally coinciding with the wavelength of either the signal or idler waves generated by the OPO, is injected into the OPO cavity. Injection seeding has two effects on the OPO. First, it can be used to force single mode operation of the OPO in much the same way as injection seeding can force single-frequency operation of a Q-switched solid state laser. If the injected power significantly exceeds the spontaneous emission into a cavity mode,oscillation will build up on the longitudinal mode that is closest in frequency to the seed source. The amount of power needed to seed a particular mode depends on the frequency difference between the seed source and a longitudinal mode of the OPO.
The second effect of injection seeding an OPO is to shorten the pulse buildup time. Due to the existence of the seeding wave, the OPO field in the cavity does not have to build up from noise. This has the effect of decreasing the threshold power and increasing the efficiency of the OPO for a pulsed pump.
The effectiveness of injection seeding depends on the intensity of the seeding wave in the cavity. If a weaker seed source is used, the efficiency and threshold advantages gained by seeding decrease significantly. The effect of injection seeding a pulsed optical parametric oscillator has been considered by a number of authors, such as A. G. Marunkov, V. I. Pryalkin and A. I. Kholoknykh, "Improvement in the Conversion Efficiency of Pulsed Optical Parametric Oscillators Using External Signal Injection," Sov. Journal of Quantum Electron. 17, 392 (1987). It has been shown that injected seeding powers of 1 watt or more can have a dramatic effect on the efficiency and threshold of a pulse-pumped OPO. Seeding power level of such magnitude can be obtained by using a Q-switched source.
Use of a pulsed seed source, however, can be difficult because it requires careful timing of the injected seeding pulses relative to the pump pulses. Furthermore, the Q-switched seed source has to be of a single frequency, which is generally difficult to build.
There are other difficulties in using injection seeding to enhance the performance of an OPO. For efficient seeding, it is necessary to spatially and spectrally match the mode of the seeding wave to the OPO cavity. The frequency of the external source must be nearly resonant with the OPO cavity. This requirement often results in optomechanical complexity. It is also necessary to optically isolate the seed source from the OPO, because the power reflected from or generated by the OPO can destabilize the seed source, or even destroy it if the power from the OPO is large.